1. Field of the Invention:
The present invention relates to the transmission of data from the bottom of a bore hole to the surface and, more particularly, to a device for creating information-carrying pressure pulses in the circulating flow of fluid between a drill bit and the surface by selectively controlling the fluid flow patterns.
2. Discussion of the Prior Art:
For reasons of economy and safety it is highly desirable that the operator of a drill string be continually aware of such down-hole parameters as drill bit position, temperature and bore hole pressure. Knowledge of the drill bit position during drilling can save significant time and expense during directional drilling operations. For safety it is of interest to predict the approach of high pressure zones to allow the execution of proper preventive procedures in order to avoid blowouts. In addition, efficient operation of the drill string requires continuous monitoring of down-hole pressure. The pressure in the bore hole must be maintained high enough to keep the walls of the hole from collapsing on the drill string yet low enough to prevent fracturing of the formation around the bore hole. In addition the pressure at the bit must be sufficient to prevent the influx of gas or fluids when high pressure formations are entered by the drill bit. Failure to maintain proper down-hole pressure can and frequently does lead to loss of well control and blowouts.
Any system that provides measurements while drilling (MWD) must have three basic capabilities: (1) to measure the down-hole parameters of interest; (2) to telemeter the resulting data to a surface receiver; and (3) to receive and interpret the telemetered data. Of these three essential capabilities, the ability to telemeter data to the surface rapidly is the current limiting factor in developing MWD systems.
Four general methods have been studied that would provide transmission of precise data from one end of the well bore to the other: mud pressure pulse, hard wire, electromagnetic waves, and acoustic methods. At this time, mud pulsing has proven to be the most practical method.
In a typical mud pulsing system pressure pulses are produced by a mechanical valve located in a collar above the drill bit. The pulses represent coded information from down-hole instrumentation. The pulses are transmitted through the mud to pressure transducers at the surface, decoded and displayed as data representing pressure, temperature, etc. from the down-hole sensors. Of the four general methods named above, mud pulse sensing is considered to be the most practical as it is the simplest to implement and requires no modification of existing drill pipe or equipment.
Mechanical mud pulsers, known in the art, are inherently slow, producing only one to five pulses per second, are subject to frequent mechanical breakdown, and are relatively expensive to manufacture and maintain. An example of such a device is disclosed in U.S. Pat. No. 3,958,217 (Spinnler) disclosing a valve mechanism for producing mud pulses.
U.S. Pat. No. 4,418,721 (Holmes) discloses the use of a fluidic valve to rapidly change the flow of mud from radial to vortical and back again, altering the flow pattern of the fluid and producing pressure pulses therein. Mud flow through the valve transits a vortex chamber and diffuser assembly in a generally radial flow pattern, exiting the valve through an outlet located at the center of the chamber on one side of the assembly. A small tab is selectively extended from a recessed position into, and retracted from, the vortex chamber by a solenoid responding to encoded sequences of electrical impulses from measurements made by down-hole sensors. The insertion of the tab into the vortex chamber disturbs the fluid flow and transforms the radial flow to vortical, producing a pressure pulse that is radiated through the mud back up the drill pipe to the surface transducer. The activation energy for the tab is relatively low and the permissible pulse rate is therefore much higher than can be achieved with mechanical valves. Disadvantageously, such devices are characterized by relatively restrictive flow channel sizes requiring parallel connection of multiple valves with accompanying energy and volume requirement penalties and clogging potential. In addition, areas within the assymetrical vortex chamber suffer high pressure and wear, necessitating frequent inspection and maintenance and requiring costly reinforced construction.